1. Field of the Invention
This invention relates to electrical devices and assemblies and methods of making such devices and assemblies.
2. Introduction to the Invention
Circuit protection devices comprising a conductive polymer composition having a positive temperature coefficient (PTC) are well-known. Such devices which are intended for surface mounting onto a substrate, e.g. a printed circuit board, are disclosed in U.S. Pat. Nos. 5,831,510 (Zhang et al), 5,852,397 (Chan et al), and 5,864,281 (Zhang et al), International Publications Nos. 94/01876 (Raychem Corporation) and 95/08176 (Raychem Corporation), and copending, commonly assigned application No. 09/181,028 (Graves et al, filed Oct. 27, 1998), the disclosures of which are incorporated herein by reference. Such circuit protection devices generally comprise first and second laminar electrodes; a laminar PTC resistive element sandwiched between the electrodes; a third (residual) laminar conductive member which is secured to the same face of the PTC element as the second electrode but is separated therefrom; and a cross-conductor which passes through an aperture in the PTC element and connects the third conductive member and the first electrode. This permits connection to both electrodes from the same side of the device, so that the device can be connected flat on a printed circuit board, with the first electrode on top, without any need for leads. The resistive element preferably comprises a laminar element composed of a PTC conductive polymer. Preferably the device comprises an additional conductive member and an additional cross-conductor, so that the device is symmetrical and can be placed either way up on a circuit board.
When two of these devices are physically secured together in a stacked configuration, a composite device can be formed. Such composite devices have the same small xe2x80x9cfoot-printxe2x80x9d on the board, i.e. occupy a small area, as a single device, but they have a lower resistance than can be conveniently produced by using a single device. In addition, the power dissipation of such a composite device is not substantially different from the power dissipation of one of the devices alone. As a result, the composite device has a lower resistance for a given hold current where xe2x80x9chold currentxe2x80x9d is the largest current which can be passed through a device without causing it to trip.
As described in copending, commonly assigned U.S. patent application No. 09/060,278 (Chiang et al, filed Apr. 14, 1998), the disclosure of which is incorporated herein by reference, composite devices can be prepared by sorting individual devices and then assembling the sorted devices into composite devices. Such a process can be tedious, as it may require that the resistance of each individual device be read. We have now found, in accordance with the present invention, that it is possible to prepare a multilayer assembly from which individual composite devices can be divided. Such an assembly allows preparation of a large number of composite devices simultaneously. Furthermore, because the process described herein allows the patterning of individual layers of the assembly before or after fabrication into the assembly, a variety of different devices can be prepared from the same starting layers. In addition, the composition of the layers can be easily varied, allowing the simple build-up of devices with combined functionality. Various interconnection schemes between layers can be simply implemented, and devices with multiple external electrical contacts can be made without changing the basic manufacturing process. All of these further add to the broad range of different devices which can be inexpensively mass-produced by the process disclosed herein.
This invention provides methods and pro for which various operative steps can be carried out on an assembly which will yield a plurality of devices when subdivided into composite devices by subdividing along both x- and y-directions (where x and y correspond to directions in the plane of the laminar PTC elements). The ability to prepare devices in this way is a significant improvement over other methods, for example described in U.S. application No. 09/060,278, because for this invention, individual devices do not need to be individually assembled, increasing the efficiency and before reducing the cost of the manufacturing process. Finally, the method of combining layers of materials to form composite devices disclosed herein allows an extremely simple yet adaptable method for forming a variety of devices without the necessity of changing the basic manufacturing process.
In a first aspect this invention provides a process for manufacturing a composite polymeric circuit protection device, said process comprising
(1) providing a polymeric assembly comprising
(a) providing first and second laminates, each of which comprises a laminar polymer element having at least one conductive surface,
(b) providing a pattern of conductive material on at least one of the conductive surfaces on one laminate;
(c) securing the laminates in a stack in a desired configuration, at least one conductive surface of at least one of the laminates comprising an external conductive surface of the stack, and
(d) making a plurality of electrical connections between a conductive surface of the first laminate and a conductive surface of the second laminate; and
(2) subdividing the stack into individual devices each of which comprises at least one electrical connection.
In a second aspect this invention provides a polymeric assembly comprising:
(a) a first laminate comprising a laminar polymer element having at least one conductive surface having a pattern;
(b) a second laminate comprising a laminar polymer element having at least one conductive surface having a pattern said second laminate being secured to the first laminate in a stack so that the stack has first and second external conductive surfaces; and
(c) a plurality of transverse conductive members which run through the first and second laminates between the first and second external conductive surfaces.
Using either the process or the assembly of the invention, devices can be made by creating electrode precursors in the form of conductive surfaces of appropriate shapes upon resistive elements which are larger than the desired final shape, forming a stack of a plurality of resistive elements which is also larger than the desired final shape, and then subdividing the stack into individual devices. Electrodes of appropriate shapes can be made by removing unwanted portions of any one, or any combination, of the conductive surfaces. The removal can be accomplished by milling, stamping, or etching, for example. Alternatively, the electrode precursors can be formed by patterning conductive material onto any one or any combination of the PTC resistive element surfaces by chemical vapor deposition, electrodeposition, sputtering, etc. Conductive material may also be applied to the faces of the PTC resistive elements by use of an adhesive or tie layer. Electrical interconnection between a desired combination of the conductive surfaces of the plurality of resistive elements can be accomplished before the stack has been subdivided into individual devices. Alternatively, some or all of the electrical connections between desired electrodes or contact points can be made after the stack has been subdivided into composite devices. The electrical interconnection can be designed so that connection is made between some of the conductive surfaces of the stack or the electrodes of the device, but not all.
Thus in a third aspect, this invention provides a composite device, which can be made, for example, using the process of the first aspect of the invention or the assembly of the second aspect, comprising
(1) first and second external laminar electrodes,
(2) third and fourth internal laminar electrodes,
(3) first and second laminar PTC resistive elements, each of which (i) exhibits PTC behavior, and (ii) comprises a laminar element composed of a PTC conductive polymer,
xe2x80x83said first resistive element having a first face to which the first external electrode is secured and an opposite second face to which the third internal electrode is secured, and said second resistive element having a first face to which the second external electrode is secured and an opposite second face to which the fourth internal electrode is secured,
(4) a fifth external laminar conductive member which is (i) secured to the first face of the first PTC resistive element, and (ii) is spaced apart from the first external electrode,
(5) a sixth external laminar conductive member which (i) is secured to the first face of the second PTC resistive element, and (ii) is spaced apart from the second external electrode,
(6) a seventh internal laminar conductive member which (i) is secured to the second face of the first PTC resistive element, and (ii) is spaced apart from the third internal electrode,
(7) an eighth internal laminar conductive member which (i) is secured to the first face of the second PTC resistive element, and (ii) is spaced apart from the fourth internal electrode,
(8) a first aperture which runs between the first external electrode of the first laminar PTC element and the second external electrode of the second laminar PTC element,
(9) a second aperture which runs between the fifth external laminar conductive member of the first laminar PTC element and the sixth external laminar conductive member of the second laminar PTC element,
(10) a first transverse conductive member which
(a) lies within the first aperture,
(b) runs between the first external electrode of the first laminar PTC element and the second external electrode of the second laminar PTC element,
(c) is secured to the first PTC element, the second PTC element and the third laminar element, and
(d) is physically and electrically connected to the first external laminar electrode, the seventh internal laminar conductive member, the eighth internal laminar conductive member, and the second external laminar electrode, but is not connected to the third or the fourth internal electrode, and
(11) a second transverse conductive member which
(a) lies within the second aperture,
(b) runs between the fifth external laminar conductive member and the sixth external laminar conductive member,
(c) is secured to the first PTC element, the second PTC element and the third laminar polymer layer, and
(d) is physically and electrically connected to the fifth external laminar conductive member, the third internal electrode, the fourth internal electrode, and the sixth external laminar conductive member, but is not connected to the first or second external electrode.